Do:
-Make an attempt to learn how to build a pc (read online guides, past posts, motherboard manual, and more!)
-Always think of all the problems you can run into with your specific part combination (e.g. google your motherboard model and video card with keywords: problems, issues, and incompatibility)
-Make a specific list of what you'd want in a pc before requesting someone to make you a list of parts (e.g gaming at 80fps on high resolutions, e-sata, overclocking, Home Theatre capability, and etc.)
-purchase your parts at a local shop since it will be easier to exchange and refund unless you're the most patient person in the world

Don't:-Rely on a forum as your tech support for your all your problems with parts... the manufacturers have a knowledge base, faq, and a phone line to address your issues!
-Do anything fancy until you've confirmed stability and performance with memtest and the usuals
-Assume building a pc is easy. It requires a lot of dexterity and a lot of time during your first build.
-Insult others for not responding to your "Hardware Problem Related" post in 2 minutes
-Ignore past posts with similar problems and hardware (THE SEARCH BUTTON EXISTS FOR A REASON!!!)

My points are focused on the minor still from highschool or junior high because we know there are more kids on the internet than adults, right?